These days, vaccines are commonly given to horses. But as common as vaccinations are questions about vaccinations. Read on to learn the hows, whys, whats, and whens of vaccinating horses.

#1 How Do Vaccines Work?

Vaccines resemble disease causing microorganisms (viruses or bacteria) which create an immune response in horse’s body to provide protection: the horse’s body is stimulated to identify and destroy the invading microorganism, and antibodies are created which allow the horse to “remember” the microorganism so it can better destroy it should it ever invade the horse’s body again.

There are two main types of vaccines: modified live vaccines and killed vaccines. Modified live vaccines contain the pathogen itself, which has been altered so that while it does replicate in the horse’s body, it does not fully infect the host with the disease. This is typically done by attenuation in cell culture, using harmless variants from other species, or creating temperature-sensitive mutants.

Killed vaccines lack pathogenicity. In horses, killed vaccines either contain the inactivated pathogen or protein components of the pathogen. Killed vaccines typically require the addition of an adjuvant to stimulate adequate immune response.Vaccine technology is continually improving, bringing new and better vaccines to help protect our horses. For many diseases there are a number of different vaccines available to choose from.

In some cases, a specific type of vaccine may be considered more effective than another. “Theoretically, live vaccines may give better protection. They may give a broader and more effective immune response,” said Dr. Townsend, DVM, Professor, Department of Large Animal Clinical Science, Western College of Veterinary Medicine, Saskatoon, Saskatchewan, a leading researcher into vaccines and vaccine efficacy in horses. “But we don’t have the head to head data that says for sure that live vaccines are better than killed. The killed vaccines have also been much improved over the last few years.”

#2 Why Vaccinate?

Vaccines serve two primary purposes for an individual horse: the first is to greatly reduce or eliminate the possibility of a horse getting a particular disease, and the second is to reduce the severity and consequences of the disease should he get it.

In addition, vaccinating a horse against communicable diseases also helps protect other horses by removing that horse as a vector for transmission.

While most vaccines are not 100 percent effective in protecting against a disease, there is still good reason to vaccinate. “If a vaccinated horse becomes exposed, it won’t become as ill as a nonvaccinent. A vaccinated horse will also shed less virus into the environment and therefore be less threat to other horses,” said Dr. Townsend. “It’s important not just to vaccinate one horse but to vaccinate the group. That makes it difficult for the virus to actually take hold within the group.”

This concept, called herd immunity, is an important concept in controlling contagious disease. Essentially, it involves immunizing a high enough percentage in a population to protect that population against the spread of disease.

“There are two reasons to vaccinate a horse: to protect the individual, but also to stimulate herd immunity, which is also important in protecting the individual,” said Dr. Townsend.

By obtaining herd immunity, both vaccinated and nonvaccinated animals have a decreased chance of being exposed to disease in the first place. It is two-fold protection for the vaccinated horse. “If you just vaccinate one animal in a large stable and the rest of the stable gets sick and are shedding huge amounts of virus, it may very well overcome the immunity of the vaccinated horse and it may get sick anyway,” said Dr. Townsend. He suggests vaccinating all horses at roughly the same time and vaccinating any newcomers to the herd before or as they arrive in order to build herd immunity.

#3 What Vaccines Should My Horse Receive?

In their “Guidelines for Vaccination” the American Association of Equine Practitioners (AAEP), the association that guides equine veterinary practices in North America, has set a number of vaccines as “core” or important vaccines for all horses in North America. Core vaccines protect against diseases that pose significant threats to horse and human health.

If your horse travels to another country or province, he may need additional vaccines.

The AAEP’s core vaccines include those for:

Tetanus — Tetanus, or “lockjaw,” is caused by a potent neurotoxin from the bacteria Clostridium tetani. As the bacteria are often present in the manure of horses and other animals, as well as soil, and can last in the environment for many years, horses are particularly susceptible to this often fatal disease. “Tetanus is considered core because it is such a devastating disease if a horse gets it,” said Dr. Townsend. “It’s rare, but the vaccine is inexpensive to give, and horses are in an environment where they can get it, so it makes sense to vaccinate against tetanus.”

Eastern/Western Equine Encephalomyelitis (EEE/WEE) — EEE and WEE are neurologic diseases transmitted by mosquitoes. EEE is 90 percent fatal, while WEE is only 50 percent fatal. Foals are considered more susceptible than adult horses.

Rabies — Though rare in horses, this neurologic disease is always fatal and of great significance to public health. Rabies is transmitted through a bite from an infected carrier, typically foxes, bats, raccoons, and skunks.

West Nile virus (WNV) — Transmitted to horses from birds via mosquitoes, WNV causes swelling of the brain and spinal cord. The disease is approximately 33 percent fatal for horses exhibiting signs of the disease and 40 percent of horses will have permanent residual effects after surviving the acute illness. Introduced to North American in 1999, WNV is now present in all Canadian provinces and US states. Many people have been concerned about the safety and efficacy of the WNV vaccine as it came on the market quickly after WNV became a problem for North American horses. However, Dr. Townsend confirmed that “the vaccines have been widely used and it’s clear that they are safe. And through field studies and through experimental studies it is clear that the vaccines are very effective. So it turned out to be an organism against which it was possible to make vaccines very quickly that were both safe and effective.”

Other non-core vaccines include those for:

Influenza — Equine influenza is one of the most common causes of respiratory tract infection in horses, causing fever, cough, and nasal discharge. Though rarely fatal, it will layup a horse for approximately three weeks and is extremely virulent.

Equine Herpesvirus (EHV) — Commonly called rhinopnuemonitis, EHV comes in two forms: type 1 (EHV-1) and type 4 (EHV-4). Infection is similar to influenza, infecting the upper respiratory tract and producing symptoms of fever, cough, and nasal discharge. Anorexia and lethargy are also common symptoms. However, EHV-1 also causes epidemic abortion in mares, the birth of weak foals that usually die, and paralytic neurologic disease. “Most of the concern is around abortion and neurologic disease,” said Dr. Townsend. “That’s where most of the research is being done. At this point, there’s no evidence that the vaccine can protect against the neurologic form of the disease, but there is some evidence that you can protect against abortion.”

Equine Viral Arteritis (EVA) — While typically not life-threatening to a healthy, adult horse, this viral disease can cause abortion in mares and death in foals. Stallions and colts can become long-term carriers of the disease, transmitting it to mares in their semen.

Strangles — Strangles is a highly contagious disease caused by the bacterium Streptococcus equi subspecies equi. Symptoms include fever, nasal discharge, and swollen lymph nodes in the head and neck, which may burst and leak pus. Spread by direct contact, strangles is an endemic disease in all horse populations.

Potomac Horse Fever (PHF) — Caused by Neorickettsia risticii, PHF has been indentified only in specific areas of the US and Canada. Symptoms include fever, diarrhea, laminitis, and mild colic.

Rotavirus — Rotavirus is a major source of diarrhea in foals. It is transmitted to foals when contaminated feces enters the foal via the mouth. The virus damages the small intestine villi resulting in maldigestion, malabsorption, and diarrhea.

Botulism — Botulism is the disease caused by the toxins of the spore-producing bacteria Clostridium botulinum. The toxin can be introduced to the horse through wounds or by eating contaminated vegetation.

Anthrax — Anthrax is a rapidly fatal septicemic disease caused by the vegetative form of Bacillus anthracis, which is acquired by the horse through ingestion, inhalation, or contamination of wounds

AAEP has labeled non-core vaccinations as “risk-based vaccines.” However, most veterinarians would agree that even core vaccines are given based on risk. “The truth is that some (vaccines) are more important in some regions than others,” said Dr. Townsend. This is why it is so important to consult a local veterinarian, who can create the best vaccination program based on the individual situation of your horse.

A horse that comes in contact with many unknown horses, such as a show horse or racehorse, or a horse living at a stable with lots of in-and-out traffic, has increased risk for contagious disease.

Geography, amount of contact with unknown horses, age, breeding status, and management are just a few of the factors to be considered when deciding what vaccines a particular horse should receive. Variables can change from year to year, so make sure to update your vaccination protocols with your veterinarian as necessary.

In a country as large as Canada, the presence of certain diseases varies greatly. Rabies, for example, is rarely seen in Western Canada. “My guess is relatively few people (in Western Canada) vaccinate against it,” said Dr. Townsend. The incidence of rabies in Eastern Canada, however, is much higher due to the presence of foxes. “I would assume that vaccination against rabies would therefore be much more in Eastern Canada,” Dr. Townsend continued. EEE, WEE, PHF, and anthrax are other diseases that are highly geographically dependent.

Geography is also an important consideration if your horse is likely to travel. A horse that travels to the USA for shows may need additional vaccines compared to a horse that stays in the same city or province.

Another important consideration is the amount of contact your horse has with unknown horses, directly or indirectly. For example, a horse that goes to competitions will have an increased risk for diseases such as influenza and strangles, and therefore vaccination may be recommended. A horse that lives at the same stable as the competition horse, even if he does not compete himself, is still indirectly exposed to many unknown horses and therefore more exposed to disease.

Age and breeding status are two other important considerations in a vaccination program. Rotavirus, for example, is common in foals but not adult horses. Broodmares are often given vaccines for rotavirus in order to protect their foals from the virus via passive immunity. Breeding stallions, particularly of certain breeds, may also be vaccinated against EVA to avoid infection and carrier status.

Finally, some management practices may necessitate further vaccines. A veterinarian may suggest, for example, that a horse whose diet consists of haylage or silage be given the botulism vaccine, as these forage types can contain the botulism bacteria.

#4 When Should I Vaccinate?

Once you and your veterinarian have decided what vaccines to give your horse, you will need to decide when to vaccinate.

Time your vaccinations so they are given a few weeks before the period of greatest risk. While springtime is typically the best and most convenient time for most vaccinations, check to make sure that vaccinating in spring really makes sense for your horse. Photo: Robin Duncan Photography

“A good idea would be to try and time vaccination with the period of highest risk,” said Dr. Townsend. “You get the best protection about two to three weeks after the vaccination is completed, and then it gradually declines after that.”

“So if you take a disease like WNV, where we know it occurs between July and October, then the obvious thing to do would be to vaccinate just prior to this time,” continued Dr. Townsend. “That would suggest that you might vaccinate in May or June.”

“If you look at a disease like influenza, horses are going to get it when they start coming into contact with lots of other horses. So vaccination prior to the show season makes more sense.”

For most diseases, it likely makes the most sense in terms of disease risk and convenience to vaccinate in the spring, but there may be some adjustments needed. “Look to make sure doing them all at one time really makes sense,” Dr. Townsend cautioned. Timing is particularly important if your horse usually receives a combination vaccine, such as the “flu-rhino,” “three-way,” or “five-way” vaccines. “Most vaccines come in combination now because of convenience and that’s what people want.

I think the main thing is to pick the combination that fits your horse’s circumstances the best and try to time the vaccination so you get protection when it’s really needed.” In some cases, it may make more sense to inoculate for single diseases at different times of the year.

Along with the question of when is “how often?” Years ago, many vaccinations required frequent boosters — as often as every 60 to 90 days — in order to maintain immunity. However, “that was back in the days,” said Dr. Townsend. “Today the vaccines that are on the market should give protection for at least six months to a year or longer.”

Most horses will require only annual boosters, though some may require boosters every six months.

A question that often comes up is why horses require yearly vaccinations for diseases that humans only require inoculation against after many years, such as with tetanus. “For some reason horses don’t maintain high antibody levels as long as people do,” said Dr. Townsend. “So if you measure people’s antibodies after vaccination for tetanus, they still have antibodies after years. Whereas for horses, they don’t appear to have the antibodies after a year. So all the vaccines for horses are registered to be given once a year. They may have protection for longer, but we don’t know that for sure, so it’s best to give vaccines yearly.”

#5 What Side Effects Can Occur?

Side effects from vaccinations are rare in horses. “Usually it’s just lumps and bumps that disappear in a few days,” said Dr. Townsend. Swelling and soreness where the vaccine was injected is due to the body producing an immune response.

However, more serious side effects can occur. “Sometimes animals have slight or severe allergic reactions to vaccines,” said Dr. Townsend. Always watch your horse for at least 30 minutes following vaccination to ensure no adverse effects have occurred. Coughing and hives are signs of an allergic reaction, and a veterinarian should be summoned immediately if one is not already present. Anaphylaxis can occur in rare cases.

Discuss what vaccines your horse should receive with your local veterinarian, who can design a vaccine program based on your horse’s unique situation. Photo: Pam MacKenzie Photography

Another serious adverse reaction is not a fault of the vaccine itself, but occurs when bacteria are introduced to the horse via a needle. This can cause an abscess to form at the vaccination site and large amounts of tissue damage can occur.

All significant adverse reactions to vaccines are reported to the Canadian Food Inspection Agency and the vaccine manufacturer. “Should a vaccine come on the market that ends up having a lot of reactions, they (the companies and government) will certainly take action,” said Dr. Townsend.

To reduce the risk of side effects, always use a new, sterile needle for each vaccine to be given. Administering multiple vaccines at the same time or in the same location may increase the chance of adverse reaction, so if more than one vaccine needs to be given, consider doing so a few days apart or in different locations on the horse. Vaccines should be given by or under the supervision of a veterinarian in case an adverse reaction does occur.

Take-Home Message

Vaccines are one of the primary ways to protect your horse and other horses against disease. Discuss with your veterinarian what vaccines are best given your horse’s specific situation, and develop a timeline for vaccination to give maximum immunity during maximum risk periods. The vast majority of vaccines have been proven safe and effective, and when used properly at the right times, provide good protection against a variety of illnesses.

Main Article Photo: Vaccinating a group of horses against contagious diseases will establish herd immunity, giving protection to vaccinated horses while helping protect the non-vaccinated horses they come into contact with.

This article originally appeared in the March 2010 issue of Canadian Horse Journal.